A bomb went off in a London subway train this morning, injuring more than 20 people. The explosion shot flames through the subway car, burning commuters. Police are calling it a terrorist attack but say the injuries are not life-threatening. This is the fourth terror attack in the British capital since March. NPR's London correspondent Frank Langfitt is following the story.

Hey there, Frank.

FRANK LANGFITT, BYLINE: Hey. Good morning, Mary Louise.

KELLY: Good morning. What more do we know about what happened on this train?

LANGFITT: This happened a little after 8 o'clock, this morning's rush hour. A London-bound District Line had stopped in a place called Fulham. It was about four miles southwest of Big Ben. The explosion sent everybody running, of course, from the car. If you look at the video online, it looks to be a homemade bomb. It was in a white plastic bucket. There were flames coming out of the top. There were also these wires that everybody noticed that were also coming out of the top of the bucket. The bucket, when it was photographed and videoed, was next to the subway train door. One passenger posted a photo of (ph) Twitter of his own face, which showed that he had a big red burn mark from the flames, and his hair was singed.

KELLY: Early hours, but do we know who might have been behind this? Any claim of responsibility?

LANGFITT: No, not that we know yet. This is still very early on. Police are on a manhunt of some sort but haven't given us much details. They're doing the usual stuff - interviewing witnesses, looking - asking for cellphone footage and going through lots of closed-circuit video, which there's a ton of here in London.

KELLY: Now, President Trump tweeted about the attack this morning. He wrote - and I'm going to quote it - "another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

Frank, that phrase there, people who were in the sights of Scotland Yard...

LANGFITT: Yeah. So...

KELLY: ...Do we know what President Trump is referring to?

LANGFITT: I do. It's very unlikely that he would have any intelligence from the U.K. so early on in an investigation like this. So he's probably responding to the news that he sees, which he often does, as we know. What he's probably referring to, though, is that in some earlier attacks, Scotland Yard did know the people who were involved. And they were sort of on their radar.

The problem for Scotland Yard is they have a ton of people - more than 3,000 - on their radar. And they're having a really difficult time now being able to pinpoint those people and predict what they're going to do. I would note also that the president didn't offer any condolences to the victims and sort of moved on to other topics. So he was kind of just, like, firing off a tweet quickly and then going on to other things.

KELLY: And then moving on.

We mentioned this was the fourth attack in London this year. This is the fifth terror attack in the U.K. But it...

LANGFITT: Yep.

KELLY: ...Sounds like, thankfully, this is not as bad as some of the earlier ones.

LANGFITT: No, it's mild by comparison. Even the police say this was a small explosion. I think there's an investigation to see if, in fact, the bomb didn't quite work the way it was planned to. Because if you can see video of it, obviously, it didn't fully explode.

KELLY: Right.

LANGFITT: The earlier attacks that we had here were on London and Westminster bridges. And there was, of course, the bombing outside of the concert in Manchester. These were huge attacks, multiple fatalities, all in famous spots. This happened in Fulham - not that well-known. It's possible - the train was bound for the heart of the city, and maybe it was meant to go off later.